Recently, various display devices have been widely studied. In particular, organic electroluminescence (EL) devices are advantageous in that they can emit light with high luminance by a lowered applying voltage, whereby much attention has been paid thereto. For example, a light-emitting device comprising organic thin layers provided by vapor-depositing organic compounds has been disclosed in Applied Physics Letters, 51, 913 (1987). This light-emitting device has a structure where an electron-transporting material of tris(8-hydroxyquinolinato) aluminum complex (Alq) and a hole-transporting material of an amine compound are disposed between electrodes as a laminate, thereby exhibiting more excellent light-emitting properties than that of conventional light-emitting devices having a single-layer structure.
A green light-emitting device disclosed in Applied Physics Letters, 75, 4 (1999) utilizes a particular ortho-metalated iridium complex, Ir(ppy)3 (Tris-Ortho-Metalated Complex of Iridium (III) with 2-Phenylpyridine), as a light-emitting material to improve light-emitting properties. Although the green light-emitting device exhibits a high external quantum efficiency of approximately 8%, which exceeds that of conventional light-emitting devices, 5%, it has been required to further improve the device with respect to luminance, light-emitting efficiency and durability.
Above-mentioned light-emitting device using Ir(Ppy)3 emits only a green light to have a narrow applicability as a display device. Thus, expectations have been high for the development of a light-emitting device that can emit light of the other color with high efficiency. Turning to blue light-emitting devices, though many devices using a distyrylarylene or a derivative thereof such as DPVBi (4,4′-bis(2,2′-diphenylvinyl)-biphenyl) have been proposed, an upper limit of external quantum efficiency has been 5%. Development of a blue light-emitting device exhibiting the external quantum efficiency exceeding 5% has been highly expected because it will contribute for the progress of a color organic EL device and a white light-emitting device excellent in the efficiency.